This invention relates to spreader mechanisms for supporting the legs of a tripod erect and is particularly although not exclusively applicable to tripods for TV, video or cinematograph cameras.
Spreaders are used for fixing the spread of tripod legs particularly where the tripod is to be used on an even ground. Such spreaders comprise a central hub with three equi-spaced radiating arms which are fixed to the legs of the tripod. The spreaders can be attached to the lower most ends of the tripod legs or maybe attached to the tripod legs at an intermediate position. In particular where the tripod has extending legs, the spreader arms may be connected to the upper members of the extending legs. The hub is provided with a mechanism for controlling the angle of spread of the arms to suit the spread of the tripod legs. Our UK patent publication No. 2298570 discloses a tripod spreader mechanism in which the hub mechanism for the spreader arms coordinates the movement of the arms and has preset positions in which the arms can be locked at different angles to the hub to provide the extension required. That functionality required a complex mechanism which was costly to produce and it is an object of the present invention to provide a much simpler mechanism having a much wider range of potential positions of adjustment of the spreader arms.
This invention provides a spreader mechanism for supporting the legs of a tripod erect, comprising a hub having a central axis, three arms pivotally mounted on the hub at positions equi-spaced around the hub to swing with respect to the hub axis between extended and folded positions, a drive mechanism coordinating movement of the arms so that all three arms remain in the same relationship to the hub axis throughout the range of movement of the arms, a locking device for locking the drive mechanism against movement at any position throughout the range of movement of the arms, means to hold the locking device released to allow adjustment of the spreader member and means to deactivate the holding means to allow engagement of the locking device to lock the arms in a selected position of adjustment.
In a preferred arrangement the locking device includes a first annular member mounted for rotation in the hub about said central axis and drivably connected by said drive mechanism to said arms to be rotated by rotation of the arms and means for holding said member locked against rotation.
In one particular arrangement the drive mechanism coordinating movement of the arms may comprises a shaft mounted for linear movement along the hub axis and the locking device acts to lock the shaft against movement in the hub to lock the arms in a selected position.
More specifically the drive connection between the arms and shaft may comprise segmental gears on the arms and outwardly facing toothed racks on the shaft with which the gears are engageble whereby linear movement of the shaft causes rotary movement of the arms.
More specifically the first rotatable annular member of the locking device may encircle the shaft and may be constrained against axial movement in the hub, the annular member being in driving engagement with the shaft whereby linear movement of the shaft causes rotary movement of the annular member and brake means are mounted for linear movement on the hub parallel to the hub axis to engage and release the annular member to lock and release the shaft.
In a further arrangement the drive mechanism between the arms and said first annular member may comprise a helical gear on the annular member and part-helical gears on the arms meshing with the helical gear of the member.
In the above arrangements the means to engage and release the first annular member may comprise a second annular member mounted in the hub for axial sliding movement with respect to the hub axis but constrained against rotation in the hub, spring means being provided acting between the hub and the second annular member biassing the second and first annular members together to lock the first member against rotation with respect to the second member, said holding means comprising means for disengaging the second member from the first member.
More specifically the first and second annular members have serrations which interengage when the plates are together to lock the first plate against rotation with respect to the second plate.
Thus the holding means may comprise a rotary collar mounted on the hub and means actuated by the collar for lifting the second member out of engagement with the first member.
The hub may include a push button release for releasing the action of the holding means.